With the continuing growth of the electronics industry, diamond finds an ever-increasing demand for use as heat sinks. The present invention relates to a diamond heat sink for use with electronic devices.
Pure diamond is the most heat conductive substance known and finds many industrial applications. Natural diamond as mined, however, varies greatly in thermal conductivity depending upon the nitrogen content. For a higher nitrogen content, diamond has a small thermal conductivity; its value at room temperature varies from about 24 W/cm.degree.C. to 6 W/cm.degree.C. depending upon the nitrogen content. Diamonds with less than 1 ppm of nitrogen are classified as Type IIa, and those having a higher nitrogen content as Type Ia. Type IIa diamonds generally have thermal conductivities higher than 20 W/cm.degree.C. and are used as heat sinks for heat-generating electronic devices such as semiconductor leasers, diodes and microwave oscillating devices.
It is well known that synthetic single-crystal diamonds can be produced from a seed crystal using a temperature gradient process (see, for instance, U.S. Pat. No. 3,297,407). Furthermore, it is known that synthetic single-crystal diamonds can be produced using a temperature gradient process wherein dissolution of the diamond seed material in the melted catalyst-solvent metal during the process is prevented and a tendency for spontaneous nucleation of diamond crystals near the diamond seed material is suppressed (see U.S. Pat. Nos. 4,034,066 and 4,073,380).
Large synthetic diamonds having a diameter of 1 mm or more are generally produced by the temperature gradient process. As depicted in FIG. 1, the temperature gradient process involves producing a temperature difference .DELTA.t between a carbon source 3 and a seed crystal 1 so as to grow a diamond crystal in a solvent metal 2. As shown in FIG. 2, the driving force for the crystal growth is a supersaturated concentration .DELTA.c induced by the temperature difference .DELTA.t. By maintaining a predetermined temperature and pressure for a period of 70 to 100 hours, this process can yield a synthetic diamond of a size of 1 to 1.5 carats. Four examples of such synthetic diamonds currently available are shown in FIG. 3. Typical crystal forms are an octahedron or an octahexahedron consisting of combinations of large (111) planes, (100) planes and very small (110) planes. Most natural diamonds are, on the other hand, octahedral or dodecahedral.
Diamond applications include wire-drawing dies, bytes, bits and heat sinks. Except for bits, these devices or tools are generally in the form of hexehedron (see FIG. 3). Therefore, in order to obtain these tools of devices, octahedral, dodecahedral or octahexahedral diamonds must be ground or cut to the final hexahedral shape. Being the hardest material known, shaping diamonds requires a tremendous amount of time and money. Furthermore, diamond is very expensive and fines (excess diamond material) produced as a result of shaping are too precious to be discarded.
Therefore, the primary object of the present invention is to provide a method of producing synthetic diamonds in a hexahedral form close to the shape of the tool or device in which it is used.
Natural Type IIa diamonds are very rare and expensive, and thus heat sinks made of this type of diamond are used only in electronic devices of high reliability and long service life, typically, communications semiconductor lasers and microwave oscillating diodes. Even mined diamonds which are classified as Type IIa vary in heat conductivity with their nitrogen content, and hence are not consistent in their thermal properties.
Diamond is typically ground on a high-speed grinding wheel made of cast iron impregnated with a paste of rape oil containing diamond powder. Wear of the diamond differs greatly from one crystal plane to another. As previously mentioned, three prominent planes in diamonds are (100), (111) and (110); they have the following wear characteristics:
TABLE 1 ______________________________________ Plane Wear Conditions of Measurement ______________________________________ (100) 12 mg/hr Peripheral speed: 40 m/sec (110) 65 mg/hr Load: 750 g (111) 1-2 mg/hr Mesh size of diamond powder: #3000 ______________________________________ Note: Each plane was ground in the "easiest" work direction.
As the table shows, a diamond can be freely ground on the (110) plane, but if it is ground on the (111) plane, only the surface of the cast iron grinding wheel wears and the diamond can hardly be ground. Therefore, finding the correct or "easy" work plane is essential for precisely shaping the diamond to the desired size. For natural diamonds which are either a dodecahedron consisting of (110) planes or an octahedron with (111) planes it is not infrequent that the edge defining two adjacent planes is lost due to melting. Thus, finding the correct plane for shaping requires a great amount of skill and is error prone. As a further problem, most planes of a diamond are curved and a significant portion of them must be removed to provide a straight surface. Obviously, this results in a waste of energy and of the precious diamond, and results in a costly heat sink.
When the surface of a diamond heat sink is used as an electrode, it must be provided with a gold coating. With the coating technology available currently, it is difficult to attain a sufficient surface strength for all heat sinks. The surface of a diamond is so highly activated that is carries a significant amount of oxygen molecules. Therefore, if gold is directly vapor-deposited on the diamond, an adhesion strength sufficient to permit bonding to a device or lead wires is not obtained. To avoid this problem, a metal such as Ti or Cr which is highly reactive with oxygen is first deposited on the diamond surface by ion plating or sputtering, and then gold is coated on that metal by either the same technique of vapor deposition. However, it is difficult to effect exact control over the conditions of the surface treatment and the Ti or Cr coating, and a sufficient adhesion strength is not readily obtainable between the diamond and the Ti or Cr coating. Moreover, if lead wires are bonded to the gold film, the primer coat (Ti or Cr) may sometimes peel off the diamond.
Using a thermal gradient process developed some ten years ago, single-crystal synthetic diamonds can be produced having only an octahexahedral or octahedral form with large (111) planes (see R. H. Wentrof, J. of Physical Chemistry, vol. 75, no. 12, 1971). This is because no sophisticated temperature control techniques were available at that time and it was not clearly understood how the form of the single-crystal diamond synthesized by the thermal gradient process correlated to the synthesis temperature.
With recent improvements in the techniques of control over the synthesis temperature, researchers have attained an in-depth knowledge about the correlation between the synthesis temperature and the form of single-crystal diamonds produced by the thermal gradient process. According to their findings, a diamond in a hexaoctahedral form which has large (100) planes, and hence is closer to a hexahedron, can be produced in a temperature range the lower limit of which is a temperature 20.degree. C. higher than the melting point of the carbon source and solvent metal system and the upper limit of which is a temperature 40.degree. C. higher than that lower limit. In a range up to a temperature 50.degree. C. higher than the upper limit, an octahexahedral crystal with large (111) planes is produced, and at even higher temperatures, an octahedron is formed. Of these forms, the one having a shape close to hexahedron is of commercial interest. However, a crude hexaoctahedral diamond having a shape close to that of a hexahedron has fairly large (111) planes, and the overall proportion of (100) planes is only 60 to 70%. Furthermore, with this crystal form, crude diamonds larger than 0.2 carat cannot be synthesized even if the process time is prolonged. The reason is that, as the diamond grows, the temperature of the growth point of its single crystal shifts toward the higher range (see FIGS. 4 and 5). In particular, the growing faces at the frontmost end (indicated at 9 and 10 in FIG. 5) come into contact with the hot solvent 2 and a large (111) plane (indicated at 10 in FIG. 5) is obtained. In other words, the more the crystal grows, the higher the temperature of the growing faces at the frontmost end, with the result that an octahexahedral diamond with large (111) planes or an octahedral diamond is more easily formed than a hexahedral diamond.
The problem with the conventional thermal gradient process is that it is unable to produce a large crude hexahedral diamond because the temperature of the seed crystal is kept constant throughout the synthesis. As mentioned earlier, most of the single crystal diamonds synthesized ten-odd years ago were in either an octahexahedral or octahedral form. The two major reasons are: in the absence of a sophisticated technique of temperature control, it is difficult to retain the temperature immediately above the melting point of the solvent metal for an extended period of time and the diamond was synthesized at higher temperature, and manufacturers attempted to keep the temperature of the seed crystal constant, neglecting the fact that the temperature of the single crystal changed as it grew.